The growth in use of portable electrically-powered devices has increased the demand for small power sources, and especially metal-air electrochemical cells. Such small cells are usually disc-like or pellet-like in appearance, and may be similar in size to garment buttons, although they can be either smaller or larger. Typical such cells generally have diameters ranging from less than 0.25 inch (6 millimeters) up to about 1.0 inch (25 millimeters), and height ranging from less than 0.15 inch (4 millimeters) up to about 0.60 inch (15 millimeters). The small size and the limited amount of electrochemically reactive material which can be contained in these small metal-air cells make it desirable to direct considerable attention to improving the efficiency and completeness of the power generating electrochemical reactions which occur in such cells.
The basic chemical reactions of such cells are known. Namely, metal-air cells convert atmospheric oxygen to hydroxyl ions in the air cathode. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc.
More particularly, the desired reaction in the air cathode of a metal-air cell involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions. The hydroxyl ions migrate through the electrolyte toward the anode, where oxidation of zinc may occur, forming zinc oxide, and liberating electrons.
In most metal-air cells, air enters the cell through an air port which extends through the bottom of the cathode can. The air port may be immediately adjacent the cathode assembly, or may be separated from the cathode assembly by an air chamber or an air diffusion member.
In any of such arrangements, the port is desirably configured and positioned to facilitate movement of air through the port into the call, and to the cathode assembly. At the cathode assembly, oxygen in the air reacts with water at the cathode assembly as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms hydroxyl ions.
A second, and undesirable function facilitated by the port in the bottom of the cathode can is related to moisture loss. In normal operation, the reaction surface of the cathode assembly is laden with electrolyte, water being a major constituent of the electrolyte. Accordingly, the water at the reaction surface of the cathode assembly has a vapor pressure, and is subject to evaporation at the reaction surface. To the extent water does evaporate at the reaction surface, moisture content of the cell is reduced, along with a corresponding reduction in efficiency of the cell.
Where moisture loss is excessive, the cell may fail before the electrochemical reaction materials have been chemically used up. Thus, there is a relationship between the amount of incoming oxygen that can be made available to the cell through conventional port configurations to enable cell operation, and the amount of moisture vapor which exits the cell through such port configurations.
When the cell is manufactured, and prior to the cell being placed into use, a tab is placed on the outside surface of the bottom of the can, covering the one or more air ports. By covering the air ports, the tab substantially controls ingress and egress of air and moisture through the air ports, thereby greatly slowing down the primary electrochemical reactions, and limiting moisture loss or addition. Thus, the tab extends the storage life of the cell. When the cell is to be used, the tab is removed, whereby air, including cathodic oxygen, freely moves through the air ports and into the cell in support of operation and use of the cell. At that stage, the cell is also increasingly susceptible to ingress and/or egress of moisture.
It is an object of this invention to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having one or more air entry ports so structured and configured, both individually and relative to each other, that the port configuration provides an improved relationship between the amount of incoming oxygen that is available to the cathode assembly to fuel the cell, and the amount of moisture lost from the cell by moisture vapor transport out through the ports.
It is another object to provide improved cathode can structure for a metal-air electrochemical cell, wherein the sum of the open area of the ports in combination is reduced while the cell limiting current is maintained at a desirably high level.
It is still another object to provide improved cathode can structure for a metal-air electrochemical cell, the cathode can having a plurality of ports, the port configuration being structured so that, in a metal-air cell made with the cathode can, oxygen is more uniformly distributed over the outer surface of the cathode assembly, while minimizing the combined open area of the ports which extend through the cathode can, and thereby reducing the amount of moisture loss through the ports.
A further object is to provide improved metal-air electrochemical cells having an increase in the ratio of the limiting current of the cell to the combined area of gaseous ingress and egress available through the port configuration.
A still further object is to provide yet smaller air ports in the bottom of the cathode can, and to provide methods of mechanically fabricating such smaller air ports.